In current electronics, consumer instruments are being increasingly miniaturized, and flexible printed circuit boards constituting such instruments are accordingly getting smaller, thinner and more compact. Therefore, such flexible printed circuit boards should become lighter, thinner and more flexible, and the expanding applications of such products are requiring higher basic performance including thermostability and electric properties of materials used to manufacture such products. For example, the application of thin and elastic flexible printed circuit boards is necessary to maintain high flexibility and elasticity.
In particular, notebook computers have recently been getting smaller and thinner because of their versatility and high performance. Accordingly, bundled shielding wires, which have been used to form interface cables contained in an optical pickup device of hard-disk drives and DVD drives, are being replaced by flatter and thinner flexible printed circuit boards. Also, the frequencies used for digital signal transmission are getting higher so that data transmission rate is higher, and this demands higher electromagnetic-wave-shielding properties than ever before.
Examples of measures for shielding against electromagnetic waves include redesigning of the pattern of wiring serving as an antenna inside a flexible printed circuit board so as to prevent unnecessary radio waves from being emitted and covering a flexible printed circuit board with a radio wave absorber. The present invention relates to the latter, in other words, the application of a noise suppression sheet onto a flexible printed circuit board.
Japanese Unexamined Patent Application Publication No. 2005-281732 discloses a noise suppression sheet that is produced by orientational dispersion of flat particles of soft magnetic metal in a binding material so as to provide electromagnetic-wave-shielding properties.
Such a noise suppression sheet is formed of an organic resin such as chlorinated polyethylene for providing flame retardancy and flexibility, and then a silicone-type, acrylic-type or another type of adhesive layer is attached onto the backside thereof for applying the sheet onto a flexible printed circuit board.
Applied onto a printed circuit board, the noise suppression sheet described in Japanese Unexamined Patent Application Publication No. 2005-281732 starts to peel off from its edges especially in an ultra-slim drive or the like in which the curvatures of the flexible portions of cables are extremely small and the operating parts move fast. In the worst case, the sheet is completely removed from the board and may fall on the pickup device of a drive so as to interfere with the operation of the drive and to cover wires on the board, thereby causing problems which could lead to the occurrence of short-circuit.
Furthermore, the measures described above require a release film for protecting an adhesive portion, thus being costly and imposing a heavy load on the environment.